There are a large number of electronic circuits, in which analog filters with adjustable frequency are applied. Examples of such a filter include bandpass filters with adjustable center frequency and band blocking filters with adjustable blocking frequency.
There are a large number of electronic circuits, in which analog filters with adjustable frequency are applied.
A typical example is circuits of measuring devices, in which these filters are used, for example, for filtering measurement signals registered with a sensor.
Among these are fill level measuring devices working with ultrasound, in the case of which ultrasonic sensors operated preferably at their resonance frequency transmit short ultrasonic wave pulses toward the fill substance and receive their echo signals reflected from the surface of the fill substance back to the sensor after a travel time dependent on the fill level. In such case, the sensor receives not only the wanted signal having the resonance frequency but also disturbance signals. The disturbance signals have, as a rule, disturbance frequencies different from the resonance frequency, and are suppressed by corresponding filtering. For this, a measurement signal is filtered out from the received signal of the sensor by means of a bandpass filter matched to the resonance frequency of the ultrasonic sensor. The resonance frequencies, however, differ from sensor to sensor and are, as a rule, dependent on temperature. In order to be able to cover the total range of possibly arising resonance frequencies, for example, bandpass filters are applied with a fixed center frequency and a very large bandwidth. In such case, disturbance signal suppression is, however, so much poorer, the larger the bandwidth of the bandpass filter.
Alternatively, adjustable bandpass filters are applied. An example is active bandpass filters with multiple cross coupling, in the case of which the center frequency is adjustable via changeable resistances. FIG. 1 shows an example of an embodiment for this. In such case, an input voltage Ui, referenced to a reference potential drops across a first resistor R1 and a first capacitor C connected thereto in series to an inverting input of an amplifier Au with constant amplification. The non inverting input of the amplifier Au lies at the reference potential. An output signal of the amplifier Au is fed back via a second capacitor C of equal capacitance to a first node located between the first resistor R1 and the first capacitor C and parallel thereto via a second resistor R2 to a second node located between the first capacitor C and the inverting input. Additionally, the first node is connected via a third resistor R3 to the reference potential. The output signal of this bandpass filter is the output voltage Uo present at the output of the amplifier Au referenced to the reference potential. This filter exhibits, however, due to the resistances, a large amount of noise, which, exactly in the case of the filtering of measurement signals, such as they exist e.g. in the case of ultrasound fill-level measuring devices, is very disturbing.
In contrast, passive filters exhibit a very much smaller amount of noise. There is, however, the disadvantage that the filter frequency can be changed only by connecting, or disconnecting, inductances and capacitances determining the filter frequency, or through the use of mechanically adjustable inductances and capacitances. Such an adjustable filter is described in DE 10 2006 052 873 A1, for example.
Core of the filter described there is an LC oscillatory circuit, which includes an inductance and a detunable capacitance, especially a varactor diode. Varactor diodes have, however, small capacitances, e.g. on the order of magnitude of some picofarad, so that these filters are only suitable for filtering signals with very high frequencies.